The invention relates to a process for the production of chipboards, fiberboards, or like boards from particles containing lignocellulose and/or cellulose and/or from other particles which are electrically nonconductive and have a poor thermal conductivity, combined with at least one binder and with water, from which a chip layer or like layer is formed on a movable support, this layer being fed to at least one high-frequency preheating zone and thereafter being compressed into boards or panels with the use of contact heat by means of a steam blast that acts from the outside toward the inside.
It is known in the manufacture of wooden chipboards that the preheating of a chip layer or mat by high frequency leads to a plasticization of the layer particles. This plasticization results, inter alia, in an improved glue utilization and, with the bulk density remaining the same, in a lowering of the glue consumption, and a shortening of the residence time of the preheated layer in the finishing press, which press can thus be equipped, starting with a board thickness of 20 mm., initially with a lower compression power ("Holz-Zentralblatt" (Central Wood Periodical) 101 (1975), 8, 84).
According to a further known process, a further reduction in the pressing time can be attained by maintaining the surfaces or cover strata of the layer in a moister condition than the inner stratum of the layer, which latter should even be relatively dry, and by increasing the pressing temperature from about 140.degree.-150.degree. C. to about 180.degree. C. This is so, because due to the difference in moisture of the layer strata and due to the increased compression temperature directly after closing of the press, steam is produced on the layer surface which from there passes, so to speak in the manner of a blast, into the less moist middle stratum so that in total, i.e. due to the high-frequency preheating step and the steam blast effect, an accelerated heating of the layer throughout is accomplished, under which condition the binder is being cured (DAS[German Published Application] 1,050,991; H. -J. Deppe and K. Ernst, "Technologie der Spanplatten" (Chipboard Technology), Stuttgart, 1964, pp. 175-177; F. Kollmann, "Holzspanwerkstoffe" (Wooden Chip Materials), Stuttgart 1977, p. 101). While the previous mode of operation involved total moisture contents of 16-18%, this moisture content was now reduced (by this further process) to about 13%, the outer strata exhibiting a moisture of 22%. The heating step, carried out in a platen press, had the effect that the boards left the press with a final moisture content of about 7-8%. The nonuniform moisture effect was achieved by spraying the surfaces of the chip cake. Thus, if the chips in the cover stratum and the chips in the middle stratum are dried for this purpose under differing conditions to varying final moisture content values, it is necessary to provide a dryer for cover stratum chips and a dryer for middle stratum chips, unless a single dryer is used to first dry the cover stratum chips and then the middle stratum chips, or to dry these chips in the reverse order. The use of two dryers does make it possible to simultaneously dry cover stratum chips and middle stratum chips, but is expensive, while the drying is time consuming.
If, in contrast thereto, the cover stratum chips and middle stratum chips are dried simultaneously in a single dryer, then the final moisture values of the middle stratum chips are higher, as experience has shown, than those of the cover stratum chips, so that such a moisture difference impairs, or can even prevent, the steam blast acting from the outside toward the inside during the finishing pressing of the layer.
It is difficult to set identical moisture values in the conventional processes for obtaining a high cover stratum moisture by spraying the lower press platens and the surfaces of the layers with water, which as surface water can be vaporized more readily and more quickly than cell-bound water. This is so, because first the lower layer stratum and thereafter the upper layer stratum makes contact with the water applied through nozzles at various points. Thereby the subsequent heating of the layer throughout becomes nonuniform, and as a consequence thereof the technological properties of the thus-manufactured boards deviate in the cover strata, which is undesirable. This disadvantage occurs to an especially marked degree if supports without sheet metal are utilized as the layer carriers, such as belts for transporting the layer and fabric belts as the press belts, for example in the form of steel mesh belts. Finally, a disadvantage of the process using very moist cover stratum chips and relatively dry middle stratum chips as the starting materials is that the cell-bound water does not evaporate as quickly as the surface water. Therefore, the steam blast effect in the press and thus the heating throughout of the layer takes place at a somewhat slower rate.
Consequently, the invention has an objective of further developing the process of the above-described type and to perfect same so that, with a minimum of expenditure a uniform, accelerated, and steam-blast-like complete heating, especially of relatively thick layers, is made possible.
This objective has been attained according to a preferred embodiment of the invention by adding to the particles to be pressed an amount of water sufficient to create a steam blast, during high-frequency preheating, that acts from the inside to the outside of the particle layer as well as for the production of an inwardly directed steam that by contact heat during finish pressing.
Thus, as contrasted to the state of the art, a characteristic of the process of the present invention is that the surfaces or cover strata of the layer are not kept moister than the layer particles of the internal stratum, before the layer passes into a high frequency heating zone; rather, care is taken that all particles are combined, prior to the formation of the layer, with a quantity of water which is sufficient so that, after producing a steam blast acting from the inside toward the outside during the high-frequency heating step, during the finishing pressing step under the effect of contact heat, a conventional steam blast acting from the outside toward the inside is still effectively produced.